Semiconductor laser source

ABSTRACT

A semiconductor laser source, including a stack of semiconductor laser diodes each including at least one active region. The active region includes a series of semiconductor layers located between an ohmic contact layer and a substrate which also assumes the function of an ohmic contact layer. Pressure keeps the diodes in contact with one another by way of their ohmic contact layers. Each diode has dimensions, especially in their thickness, so that the transient heating in each diode is as small as possible and so that the average heating in the stackable diodes does not exceed a predetermined value.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a semiconductor laser source and, inparticular, to a structure of elementary modules assembled by mechanicalclamping.

2. Discussion of the Background

Since the 1970s, it has been proposed to exploit the advantageousproperties of laser diodes (ease of fast direct modulation, highefficiency in the conversion of electrical energy into light energy in anarrow spectral band etc.) to produce devices capable of generating highpower densities per unit area. To this end, monolithic elementary laserdiodes were assembled by soldering in the form of stacks, as schematizedin FIG. 1 in the case of four elementary diodes. It can be seen that,all other things being equal, the emitted power density is inverselyproportional to the height H of the elementary diode.

Devices of this type were marketed in the 1970's by the company LaserDiode Laboratories, with a number of variants, differing by the numberof stacked elements and the number of assembled stacks. The elementarylaser diodes were single-heterojunction diodes which had a thresholdcurrent density of the order of 10⁴ A/cm², which entailed operation inshort pulses (typically 100 ns) with low recurrence frequencies(typically 1 kHz). Assemblies of this type could deliver peak powers ofseveral kW; one intended application, among others, was the pulsedillumination of scenes in the near infrared.

The development of quantum-well laser diodes in the 1980s made itpossible to improve considerably their properties, threshold current anddifferential efficiency, and consequently to increase the energyconversion efficiency, which may be up to 50% in long pulse operation(several hundreds of μs) or in continuous operation.

These new characteristics have stimulated the application of laserdiodes to the pumping of solid-state lasers, in particular YAG:Neodymiumlasers, by replacing flash or other lamps, with an increase in the"take-up" efficiency of these lasers by a factor of more than 10. Thisincrease is essentially due to the small spectral width (3 nm) of thelaser diodes, compared with that of "white" sources. The typicaloperating mode, referred to as "quasi-continuous" or QCW, consists of"long" pulses (a few hundred μs), and it is also beneficial to increasethe recurrence frequency beyond the 100 Hz obtained with flash lamps.Products are marketed, for example by the company Spectra Laser Diodes,in the form of hybrid stacks as described in the document J. G. Endrizet al., High power diode laser arrays, IEEE J. Quantum Electron., 28(4),952-965, April 1992, the design of which has the clear aim of allowingoperating at High frequency, and consequently at high mean power. Tothis end, monolithic diodes in arrays, typically of 1 cm width, aresoldered onto supports made of a material with high thermalconductivity, in the form of elementary modules which are themselvesassembled by soldering on a common support, in a number depending on thepower to be obtained, and are electrically connected in series.

This type of assembly has two drawbacks:

the cost of the assembly is high;

the emitted power density is geometrically limited by the height of anelementary module.

The object of the invention is firstly to overcome the drawback of thecost for certain applications, pumping solid-state lasers, which requirelarge numbers of stacks and will consequently be economically viableonly with a substantial reduction in the fabrication costs, but do notrequire pulsed operation whose recurrence rate is high. The inventionfurthermore has the advantage of allowing an increase in the powerdensity emitted.

The cost can be analyzed into two components.

1. The "front end", all the collective technologies (material epitaxy,microlithography, electrical contact metallization), has a cost perelementary device which decreases very rapidly as the quantitiesproduced increase, as found throughout the history of silicontechnology. This will inevitably be the case for diode laser arrays.

2. The cost of the "back end", all the assembly and encapsulationtechnologies, therefore becomes increasingly dominant as the functionsfulfilled become more complex. This is the case for the stacks marketedtoday.

SUMMARY OF THE INVENTION

The invention therefore aims to reduce the cost of the assembly bysimplifying and optimizing the elementary modules, and by simplifyingtheir assembly. It has the advantage of reducing the height H of theelementary module, and consequently of allowing an increase in the peakpower density emitted in pulsed mode.

The invention therefore relates to a semiconductor laser source,including a stack of semiconductor laser diodes, each including at leastone active region consisting of a series of semiconductor layers locatedbetween an ohmic contact layer and a substrate which also assumes thefunction of an ohmic contact layer, characterized in that it includespressure means which keep the diodes in contact with one another viatheir ohmic contact layers.

BRIEF DESCRIPTION OF THE DRAWING

The various subjects and characteristics of the invention will emergemore clearly from the following description and from the appendedfigures, in which:

FIG. 1 represents a stack of laser diodes according to the prior art;

FIG. 2 represents a simple module of a laser diode;

FIG. 3 represents a laser diode module with which a thermal dissipationsheet is associated;

FIG. 4 represents a simplified illustrative embodiment of a stack oflaser diodes according to the invention;

FIGS. 5a-5b represents curves of variations, as a function of time, ofthe current injected into a laser diode and of the variation intemperature of the active region of the diode;

FIG. 6 represents curves of variation, as a function of the injectedcurrent, of the peak power of the diode and of the bottom basedtemperature;

FIG. 7 represents curves, as a function of the injected current, of thewavelength of the light emitted by the diode and of the spectral width;

FIG. 8 represents a steady-state mean temperature profile in a laserdiode stack according to the invention;

FIG. 9 represents a change in the temperature in a laser diode hybridmodule during a pulse of duration τ;

FIG. 10 represents an illustrative embodiment of a mounting for a stackof laser diodes according to the invention;

FIG. 11 represents an alternative embodiment according to the invention.

DISCUSSION OF THE PREFERRED EMBODIMENTS

With reference to FIGS. 2 to 4, the structure of an illustrativeembodiment of a stack of laser diodes according to the invention willtherefore be described.

The elementary laser diodes used in the context of the invention aremodules which are simplified compared to those used in the prior art.

According to FIG. 2, a laser diode includes simply a monolithicsemiconductor array dimensioned as will be described below. Thisstructure will be denoted as a "simple modules". An array of this typeincludes, on a substrate S of semiconductor material, an active regionL1 which consists of a series of layers and is covered with an ohmiccontact layer C1. The substrate S may also fulfill the role of an ohmiccontact layer.

According to FIG. 3, the semiconductor array is associated with athermal dissipator TH, which is in the form of a sheet with highelectrical and thermal conductivities (made of semiconductor material,silicone, silicon carbide). Its dimensions and its characteristics willbe described below. This structure will be denoted as a "hybrid module".The dissipation sheet TH has an area which is much greater than that ofthe module, so as to make it possible to remove heat out from themodule.

In a preferred embodiment, the main faces of the arrays are covered withlayers of gold C01, C02 (for example electrolytic gold).

According to the invention, a number of modules such as the ones in FIG.2 or FIG. 3 are stacked to form a laser source L, as represented in FIG.4. They are held by clamping between two conductive parts P2 and P3. Thepart P2 is pressed against the laser L by the screw V which bears on thepart P1. The parts P1 and P3, and therefore the parts P2 and P3, areelectrically isolated from one another by an insulator DI.

The parts P2 and P3 therefore make it possible to supply the excitationcurrent to the stack L of laser diodes.

A number of variants may be envisaged regarding the means for clampingthe stack L.

The gold layers previously provided on the main faces of the modulesallow better contact between the modules. The latter are therefore notsoldered together.

During mounting, the parts P2 and P3 are separated, and the pretestedelementary modules are successively inserted into position, then thejaws are tightened and the modules are both held mechanically andelectrically connected so as to be supplied in series. This device hasthe advantage that it can be disassembled for possible modularreplacement. The geometry of the jaws may be designed so as to positionthe modules parallel by their rear face bearing on suitable stops.

Solid-state laser pumping applications require that the selectedoperating mode does not lead to spectral broadening greater than thewidth of the useful absorption band of the ion used, for example 3 nmfor the neodymium ion in the YAG. However, the wavelength emitted by alaser diode varies with the temperature of its active region with acoefficient whose order of magnitude is

    dλ/dT=0.3 nm/ ° C.

for the 808 nm wavelength used for the neodymium ion. FIG. 5schematically represents the variations of the injected current (5a) andof the active region temperature (5b) for pulse mode excitation: fromthe above considerations, it results that the temperature excursionΔT_(c) should not exceed 10° C. in the example chosen. The devicesaccording to the invention are optimized to this end.

It is assumed that an active array consists of a substrate (for exampleGaAs) on which a set of epitaxial layers is deposited, which constitutesthe active region and is optimized. The parameters to be determined arethe thickness of the substrate of an active array, the nature andthickness of the thermal dissipator, in the case of a hybrid stack, aswell as the number of modules.

The parameters characterizing the structure according to the inventionwill now be determined so as to satisfy mean and maximum transientheating temperatures.

A stack of modules is considered, which may be

either simple modules, each consisting of a monolithic diode laserarray;

or hybrid modules consisting of a diode laser array associated with aradiator array made of a thermally conductive material (silicone,silicon carbide, copper-tungsten, etc.).

A wedge, made of a suitable material, may be interposed between theupper face of the stack in order to symmetrize the assembly.

The outer faces of the extreme modules are assumed to be kept at aconstant reference temperature.

Excitation Mode

The case of a pulsed mode with low duty ratio is addressed:

injected current of the order of 120 A, providing an optical power of100 W per array, a maximum value which does not damage the device;

voltage of 2 V across the terminals of the active region of each array;

pulse duration of about 100 μs;

recurrence frequency of less than 100 Hz, so that the temperature hastime to equilibrate in the structure between two successive pulses.

Mean temperature

The steady-state mean temperature profile, along an axis perpendicularto the plane of the modules, is schematized in FIG. 8. An approximateexpression may be given for ΔT_(m), the maximum value of this heatingobtained at the centre of the structure, for a hybrid stack. Thisexpression is valid so long as the number of modules is more than a fewunits:

    ΔT.sub.m =Fτf(H.sub.1 /K.sub.1 +H.sub.2 /K.sub.2)N(N+2)/8 (1)

F: Peak heat flux generated in the active region of each array, forexample 2 kW/cm2, corresponding to pulses with peak electrical power of300 W, assuming an energy conversion efficiency of 33% and an array areaof 0.1 cm².

τ: Duration of the pulses, for example 100 μs.

f: Repetition frequency of the pulses, for example 10 Hz.

H₁ and H₂ : Respective thicknesses of an array and of the associateddissipator. These thicknesses are equal to half the thickness of thesubstrate for a simple stack, for example 100 μm.

K₁ and K₂ : Respective thermal conductivities of the materials inquestion. K₁ =0.46 W/cm/° C. for GaAs and K₂ =4.5 W/cm/° C. for SiC.

N: Number of elementary modules, N=10 for example.

With these data, and in the case of a simple stack, ΔT_(m) =1.3° C. isobtained, which is negligible compared to the amplitude of the thermaltransient considered next.

Transient Temperature

For the duration of an excitation pulse, a transient profile is added tothe previous mean profile, which transient profile changes with time asshown in FIG. 9, which represents one period of the periodic structureconstituted by the stack. So long as the thicknesses of the materialsare greater than the thermal diffusion lengths: ##EQU1##

(50 μm for τ=100 μs in GaAs), the materials where the heat flows may beconsidered as infinite, and the temperature rise ΔT_(c) of the activeregion at the end of a pulse of duration τ is given by: ##EQU2##

K,ρ and C are respectively the thermal conductivity, density andspecific heat capacity of the two materials located on either side ofthe active region. In the case of a simple stack, the materials 1 and 2are GaAs.

Under the excitation conditions already defined, the following resultsare obtained:

Simple stack: ΔT_(c) =12° C., which is a maximum value not to beexceeded;

Hybrid stack with SiC radiator array: ΔT_(c) =5° C.

The formulae given previously make it possible to optimize the design ofa stack (thicknesses, number of arrays, etc.) depending on a particularapplication, for example the desired emitted power and excitation mode.

For example, a maximum transient temperature value ΔT_(c) is fixed. Withthe aid of formula (3), a pair of values F (heat flux in the activeregion) and τ (duration of a pulse) is determined. Choosing, forexample, a value F, a pulse duration τ is determined. Next, with the aidof formula (2), the minimum thicknesses of the materials enclosing theactive region are determined.

ΔT_(c) is the minimum peak heating which can be obtained. Reducing thethicknesses of the substrate and of the dissipator below the thermaldiffusion length for the desired pulse duration would increase ΔT_(c) ;increasing these thicknesses would not reduce ΔT_(c) but would reducethe emitted power density.

By way of example, a stack of 10 elementary arrays (simple modules) isconsidered, which are arranged according to the techniques of the artand in which the dimensions of each array are as follows:

thickness of the GaAs substrate: 100 μm

width: 1 cm

length (in the direction of the laser emission): 1 mm

emitted optical power: 100 W for a current of 120 A at 2 V

In pulsed mode (100 μs, 10 Hz), the mean heating is negligible and thepeak heating is 12° C., which is acceptable for a number ofapplications. The emitted power is 1 kW for an effective area of 0.1cm².

If structures are considered in which the heat is removed solely via theparts holding the modules by clamping, and via the sides of the modules,the recurrence frequency of the pulses should be limited, for example,towards 100 Hz, for 100 μs pulses and a stack of 10 simple modules.

Structures have been considered in which the heat is removed solely viathe parts holding the modules by clamping. This limits the recurrencefrequency of the pulses towards 100 Hz, for 100 μs pulses and a stack of10 simple modules.

Hybrid modules may also be designed, in which the dissipator is longerthan the monolithic array and whose rear part extends beyond the stackand acts as a fin cooled by circulation and/or vaporization of fluid(FIG. 3). This type of structure does not substantially reduce thetransient heating, which is limited by the heat transport perpendicularto the plane of the modules, but it makes it possible to improve themean cooling. A choice may be made:

either to increase the recurrence frequency;

or to increase the number of modules in the stack, in comparison to thevalues in the example chosen.

A device making it possible to clamp a stack was produced as representedin FIG. 10.

The characteristics of a stack of 5 arrays are given in FIGS. 6 and 7,which respectively show, for a 100 μs pulse mode at 30 Hz;

the P/I characteristic (emitted power as a function of injected current)

the change in the spectral width as a function of the injected current.

These curves indeed satisfy the orders of magnitude evaluated above.

It includes a base P3 on which a laser diode stack L is placed. Afastening part P2 slides on a fixed part P1 and, with the aid of ascrew, makes it possible to fasten the stack L. The fixed part P1 isfixed to the base P3 by a screw V2, and is insulated from the base P3 byan insulating plate DI.

FIG. 11 represents a simplified alternative embodiment, in which thefastening part P2 is a spring, which is fixed to the fixed part P1 andcompresses the diode stack L against the base P3. The fixed part P1 isfixed to the base P3 by an insulating sheet DI.

We claim:
 1. Semiconductor laser source, including:a stack ofsemiconductor laser diodes, each including at least one active regionlocated between an ohmic contact layer and a substrate which alsoassumes the function of an ohmic contact layer; and a pressure deviceconfigured to keep the diodes in contact with one another so that theirohmic contact layers are in direct contact with one another.
 2. Lasersource according to claim 1, wherein the substrate comprises a materialcharacterized by a thermal diffusion length and has thickness greaterthan or equal to the thermal diffusion length.
 3. Laser source accordingto claim 2, wherein the thermal diffusion length is given by: ##EQU3## τbeing the duration of a pulse; K, ρ and C being respectively the thermalconductivities, density and specific heat capacity of the substrate. 4.Laser source according to claim 1, wherein for a value of a rise intemperature ΔT_(c) occurring at an active region upon application of apulse to said diodes, one of the values of a pair of parameters F and τis determined as follows: ##EQU4## in which: F: peak heat flux generatedin the active region of each diodeτ: duration of the current pulse K,and K₂, ρ₁ and ρ₂, C₁ and C₂ : respectively the thermal conductivities,densities and specific heat capacities of materials enclosing the activeregion; then, on the basis of the value of one parameter F or τ, thevalue of the other parameter is determined; the value of τ making itpossible to determine the thickness of the materials enclosing theactive region of a diode.
 5. Laser source according to claim 1, whereinthe active region is based on GaAs.
 6. Laser source according to claim1, wherein the pressure device comprises two pressure parts electricallyinsulated from one another and mechanically coupled, one of them beingmobile relative to the other.
 7. Laser source according to claim 1,wherein the ohmic contact layers are supplemented with electrolyticgold.
 8. Laser source according to claim 1, wherein the stack of laserdiodes is free from solder or brazing agent.
 9. Laser source accordingto claim 1, wherein thicknesses of materials located on either side ofthe active region of each diode are less than respective thermaldiffusion lengths in the materials.
 10. Laser source, including:a stackof semiconductor laser diodes, each including at least one active regionlocated between an ohmic contact layer and a substrate which alsoassumes the function of an ohmic contact layer; and a pressure deviceconfigured to keep the diodes in contact with one another via theirohmic contact layers; wherein for a maximum value ΔT_(m) of heatingoccurring at a center of the stack of laser diodes upon application ofcurrent pulses thereto, the number N of stacked diodes is given by theformula:

    ΔT.sub.m =Fτf(H/K)N(N+2)/8

with: F: peak heat flux generated in the active region of each diode; τ:duration of the current pulses; f: repetition frequency of the currentpulses; H: thickness of the substrate; K: thermal conductivitv of thesubstrate; N: number of stacked diodes.
 11. Laser source comprising:astack of semiconductor laser diodes including an active region and anelectrically conductive substrate; a pressure device configured to keepthe diodes in contact with one another; and a semiconductor thermaldissipation sheet located between each diode, said dissipation sheethaving a high thermal conductivity, being electrically conductive, andhaving an area in the plane of the diodes greater than that of thediodes; wherein the thicknesses of the materials located on either sideof the active region of each diode are greater than or equal to thethermal diffusion length in each material.
 12. Laser source according toclaim 11, wherein the thermal dissipation sheet is made of SiC or Si.13. Laser source according to claim 11, wherein the thermal diffusionlength in each material is given by: ##EQU5## τ being the duration of apulse; K, ρ and C being respectively the thermal conductivities, densityand specific heat capacity of the materials.
 14. Laser source accordingto claim 11, wherein for a value of a rise in temperature ΔT_(c)occurring at an active region upon application of a pulse to saiddiodes, one of the values of a pair of parameters F and τ is determinedas follows: ##EQU6## in which: F: peak heat flux generated in the activeregion of each diodeτ: duration of the current pulse K₁ and K₂, ρ₁ andρ₂, C₁ and C₂ : respectively the thermal conductivities, densities andspecific heat capacities of the materials enclosing the active region;then, on the basis of the value of one parameter F or τ, the value ofthe other parameter is determined; the value of τ making it possible todetermine the thicknesses of the materials enclosing the active regionof a diode.
 15. Laser source comprising:a stack of semiconductor laserdiodes including an active region and an electrically conductivesubstrate; a pressure device configured to keep the diodes in contactwith one another; and a semiconductor thermal dissipation sheet locatedbetween each diode, said dissipation sheet having a high thermalconductivity, being electrically conductive, and having an area in theplane of the diodes greater than that of the diodes; wherein for amaximum value ΔT_(m) of heating occurring at a center of the stack ofdiodes upon application of current pulses thereto, the number N ofstacked diodes is given by the formula:

    ΔT.sub.m =Fτf(H.sub.1 /K.sub.1 +H.sub.2 /K.sub.2)N(N+2)/8

with: F: peak heat flux generated in the active region of each diode; τ:duration of the current pulses; f: repetition frequency the currentpulses; H₁, H₂ : respective thicknesses of a diode and of a thermaldissipation sheet; K₁, K₂ : respective thermal conductivities of thematerials of the diode and of the thermal dissipation sheet; N: numberof stacked diodes.
 16. A semiconductor laser source comprising:a stackof semiconductor laser diodes including an active region and anelectrically conductive substrate; a pressure device configured to keepthe diodes in contact with one another; and a semiconductor thermaldissipation sheet located between each diode, said dissipation sheetbeing thermally and electrically conductive, and having an area in theplane of the diodes greater than that of the diodes; wherein thesubstrate comprises a material characterized by a substrate thermaldiffusion length and has a thickness greater than or equal to thesubstrate thermal diffusion length; and the dissipation sheet comprisesa material characterized by a dissipation sheet thermal diffusion lengthand has a thickness greater than or equal to the dissipation sheetthermal diffusion length.
 17. Laser source according to claim 16,wherein for a value of a rise in temperature ΔT_(c) occurring at anactive region upon application of a pulse to said diodes, one of thevalues of a pair of parameters F and τ is determined as follows:##EQU7## in which: F: peak heat flux generated in the active region ofeach diodeτ: duration of the current pulse K₁ and K₂, ρ₁ and ρ₂, C₁ andC₂ : respectively the thermal conductivities, densities and specificheat capacities of the materials enclosing the active region; then, onthe basis of the value of one parameter F or τ, the value of the otherparameter is determined; the value of τ making it possible to determinethe thicknesses of the materials enclosing the active region of a diode.18. A method for optimizing a semiconductor laser source having stackeddiodes held together by a pressure device, comprising:calculating aduration (τ) of a current pulse applied to said stacked diodes using:##EQU8## where ΔT_(c) is a rise in temperature in a diode's activeregion upon application of a current pulse thereto, F is a peak heatflux generated in the active region, K₁ and K₂ are respectively thermalconductivities of first and second materials enclosing the activeregion, ρ₁, and ρ₂ are respectively the densities of the first andsecond materials, and C₁ and C₂ are respectively the specific heatcapacities of the first and second materials; calculating respectivethermal diffusion lengths L₁ and L₂ of the first and second materialsusing: ##EQU9## setting thicknesses of the first and second materialsenclosing the active region greater than or equal to the thermaldiffusion lengths L₁ and L₂ of the first and second materials.
 19. Amethod for optimizing a semiconductor laser source as recited in claim18, wherein the first and second materials are the same material.
 20. Amethod for optimizing a semiconductor laser source as recited in claim18, wherein the first material comprises GaAs.
 21. A method foroptimizing a semiconductor laser source as recited in claim 18, whereinthe second material comprises Si or SiC.
 22. A method for optimizing asemiconductor laser source as recited in claim 18, furthercomprising:calculating a number N of stacked diodes using:

    ΔT.sub.m =Fτf(H.sub.1 /K.sub.1 +H.sub.2 /K.sub.2)N(N+2)/8,

where ΔT_(m) is a mean rise in temperature occurring at a center of thestack of diodes upon application of current pulses thereto and f is therepetition frequency of the pulses.